The Hydroclimate Impacts of Human-Induced Soil Degradation

Human landuse and agriculture have degraded much of the world’s soils (Lal 2015; Sanderman et al. 2017). The magnitude of degradation - quantified here by soil organic carbon (SOC) losses - varies regionally, but total aggregated losses are estimated to be ~15% of CO₂ emissions over the past 150 years (Lal 2004). SOC changes primarily impact nutrient cycling and vegetative growth. Carbon losses can also degrade soil structure and alter surface and sub-surface hydrology, particularly impacting key soil hydraulic parameters such as water content and retention, matric potential, and hydraulic conductivity (FAO and ITPS 2015). However, there still exists uncertainty regarding the magnitude of these impacts and their implications for vegetation-climate interactions (Minasny and McBratney 2018). Nevertheless, in some regions even small deficits in seasonal precipitation can incite and/or exacerbate climatic dry periods (Dai 2011). As such, there is a need to quantify how human-induced SOC changes can modulate regional hydroclimate conditions, and better contextualize these impacts with respect to drought dynamics and historical climate variability. Furthermore, while many ESMs now incorporate terrestrial carbon and nitrogen cycling (e.g. Drewniak et al. 2015), there exist few studies evaluating the hydroclimate impacts of human-induced SOC changes.

We address these needs using a set of novel global climate model sensitivity experiments that explicitly incorporate SOC in soil hydraulic parameter calculations. Specifically, we use the coupled atmosphere-ocean NASA GISS ModelE2 to conduct three experiments with modern boundary conditions: 1) a “no landuse” scenario in which SOC reflects pre-agricultural conditions ~10,000BC (NoLU), 2) a scenario where SOC reflects 2010 landuse (crop+pasture) (SOC2010), and 3) a uniform 80% SOC reduction across agricultural regions, an approximate maximum amount estimated for the most severely degraded areas (SOC80) (Lal 2004; Sanderman et al. 2017). The NoLU and SOC2010 experiments utilize SOC and textural datasets created by state-of-the-art machine-learning techniques detailed in Sanderman et al. (2017) and Hengl et al. (2017), respectively. The SOC80 experiment serves as an “upper limit” to help us assess at what magnitudes do SOC changes induce significant hydroclimate impacts.

As an example of how these SOC scenarios impact our input soil hydraulic parameters, Figure 1 shows the changes (relative to NoLU) in saturated water content and hydraulic conductivity corresponding to SOC2010 and SOC80. Under SOC2010 conditions, major European and East Asian agricultural zones show between 5-10% declines in these parameters, which could lead to potential reductions in plant and crop water availability. Decreases in these parameters are amplified under SOC80 conditions, exceeding 10-15%+ across most areas.

We present our experimental results showing how these SOC changes impact critical soil moisture thresholds, surface energy fluxes and partitioning, and drought dynamics across major agricultural regions. We further explore how SOC changes affect regional resiliency to droughts of varying intensities and durations. This study motivates the needs to better understand the role of SOC in regional hydroclimate dynamics, particularly in relation to future landuse trajectories and climate change, and improve these representations in ESM frameworks.

References:

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Drewniak, B. A., U. Mishra, J. Song, J. Prell, and V. R. Kotamarthi, 2015: Modeling the impact of agricultural land use and management on US carbon budgets. Biogeosciences, 12, 2119–2129, doi:10.5194/bg-12-2119-2015.

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Sanderman, J., T. Hengl, and G. J. Fiske, 2017: Soil carbon debt of 12,000 years of human land use. Proc. Natl. Acad. Sci.,. http://www.pnas.org/content/114/36/9575.full.pdf (Accessed August 28, 2017).